The general structure of a high speed wireless network utilizes a central Hub and a number of End User Nodes (EUNs). The basic operating unit of the system is a cell, where the Hub is located at the center of the cell and the EUNs are distributed within the coverage range of the Hub. In the downstream direction (from the Hub to the EUNs), the communication channel is a broadcasting channel. In other words, any signal sent by the Hub is received by every EUN in the system.
In the upstream direction (from the EUNs to the Hub), each EUN transmission is a unicast heard only by the Hub. In a conventional high speed wireless network, the EUNs are not configured to receive transmissions from other EUNs. The upstream communication channel is shared by many EUNs using one of two types of access systems. A first access system is known as a time division multiple access (TDMA) based system is one in which all the EUNs transmit using the same frequency, but avoid colliding by transmitting at different times. A second access system is called a frequency division multiple access (FDMA) system is one in which each EUN has a separate frequency assignment to avoid collision when coordinating transmission. In both these access systems, the downstream signal is transmitted at a different carrier frequency than the upstream signal. However, because the upstream and downstream slots are of the same length, both channels have the same bandwidth.
As the EUNs are switched on, the Hub executes a one time ranging operation. Ranging determines the distance between each EUN and the Hub. An EUN which is far from the Hub must transmit faster than an EUN which is near to the Hub. After the ranging is complete, the EUNs are accordingly aligned by the Hub in time slots from the frame of reference of the Hub.
During any given time slot, only one EUN is allowed to transmit at a time. To coordinate EUN transmission in a given time slot, the Hub may utilize one of two main coordinating modes.
A contention mode is utilized to coordinate transmission when there is a relatively small number of EUNs in the system trying to capture the upstream bandwidth. When there is a larger number of EUNs in the system, a request and grant, or piggybacking mode is used.
In the contention mode, the Hub broadcasts which EUNs are eligible to contend for a particular time slot. Each contending EUN will then answer the broadcast if that particular EUN wishes to transmit. This mode is particularly useful when there is a relatively small amount of EUNs in the system, or when idle EUNs require a route to request grants quickly.
The request and grant, or piggybacking mode functions oppositely of the contention mode. In the piggybacking mode, each individual EUN that wishes to transmit will unicast a request for a time slot from the Hub. The Hub will then broadcast a time slot grant signal to every EUN. When this occurs, each individual EUN will receive the signal and will recognize whether it, or another EUN is receiving a time slot grant. These grants will assign EUNs to time slots based on bandwidth availability. This mode is most effective when there are a large number of EUNs transmitting continuously.
In the case of a TDMA system, because all of the upstream EUN transmissions are sharing a frequency bandwidth, a MAC is needed to coordinate or assign time slots so that transmission in the upstream direction is not garbled. In other words, a MAC is needed to ensure that two EUNs are not transmitting at the same time.